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Highlights from the ATLAS Experiment Available online at www.sciencedirect.com Nuclear Physics A 982 (2019) 8–14 www.elsevier.com/locate/nuclphysa XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018) Highlights from the ATLAS experiment Iwona Grabowska-Bold on behalf of the ATLAS Collaborationa, aAGH University of Science and Technology, Krak´ow, Poland Abstract This report provides an overview of the new results obtained by the ATLAS Collaboration at the LHC, which were presented at the Quark Matter 2018 conference. These measurements were covered in 12 parallel talks, one flash talk and 11 posters. In this document, a discussion of results is grouped into four areas: electromagnetic interactions, jet quenching, quarkonia and heavy-flavour production, and collectivity in small and larger systems. Measurements from the xenon-xenon collisions based on a short run collected in October 2017 are reported for the first time. Keywords: ATLAS experiment, quark-gluon plasma, photon-induced processes, jet quenching, quarkonia production, heavy-flavour production, collectivity in small systems, xenon-xenon collisions 1. Introduction In addition to proton-proton (pp) physics, the ATLAS Collaboration [1] participates in the heavy- ion (HI) programme which has been carried out at the LHC since 2010. Lead-lead (Pb+Pb) and proton- lead (p+Pb) collisions were provided at the centre-of-mass energies of 2.76 TeV, 5.02 TeV for Pb+Pb, and 5.02 TeV for p+Pb. In October 2017 a short period with xenon-xenon (Xe+Xe) collisions was taken. This opened up an opportunity of studying impact of different geometries on a broad range of observables. More- over, the HI programme is supplemented with measurements in the pp system, which serve as a reference to disentangle initial- from final-state effects. 2. Electromagnetic interactions in the QGP The strong electromagnetic field associated with highly boosted nuclei at the LHC can be utilised to study the scattering of the quasi-real photons emitted coherently from the nuclei as they pass by next to each other. These are so-called Ultra-Peripheral Collisions (UPC). In the previous measurements of exlusive production of di-muon pairs [2] and light-by-light scattering [3], ATLAS already demonstrated that photon https://doi.org/10.1016/j.nuclphysa.2018.08.024 0375-9474/© 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). I. Grabowska-Bold / Nuclear Physics A 982 (2019) 8–14 9 fluxes emerging from Pb beams are well modelled by the STARlight generator. Also in those measurements, Δφ an acoplanarity distribution (α = 1 − π , where Δφ is a distance in azimuth between final-state particles) was measured and proved to be powerful in discriminating between signal and background processes. Very recenty ATLAS has explored a potential of observing events from exclusive production of γγ → μ+μ− contributing to a sample of inelastic minimum-bias Pb+Pb collisions [4]. After evaluating and re- moving the contribution from background sources, the azimuthal angle (Δφ) and transverse momentum (pT) correlations between the muons are measured as a function of collision centrality. Figure 1 shows the background-subtracted acoplanarity distribution in different centrality intervals. Each distribution is normalised to unity over its measured range. The > 80% distribution with a significant contri- bution from UPC events is plotted in each panel for comparison. A clear, centrality-dependent broadening is seen in the acoplanarity distributions when compared to the > 80% interval. The corresponding distribution from the γγ → μ+μ− MC samples is also shown. The MC α distributions show almost no centrality depen- dence, indicating that the broadening evident in the data is notably larger than that expected from detector effects. One potential source of modification is the final-state interaction of the produced leptons with the electric charges in the QGP. Assuming that the broadening of the α distribution results from transfers of a small amount of pT to each muon, in the 0–10% centrality interval that scale, assumed to be the RMS momentum transfer to each final-state muon in the transverse plane, is evaluated to amount to 70 ± 10 MeV. Fig. 1. Background-subtracted acoplanarity distribution (α) in four centrality intervals in Pb+Pb data [4]. A comparison to the STARlight calculation for γγ → μ+μ− is also shown. The distributions are normalised to unity over their measured range. 3. Jet quenching Using the large statistics of the 2015 Pb+Pb data, ATLAS finalised measurements of inclusive jet nuclear modification factor (RAA) [5], as well as jet fragmentation functions [6]. Those results provide very detailed studies of inclusive jet production as a function of jet pT, rapidity (y) and centrality in comparison to the reference pp data collected at 5.02 TeV. The RAA evaluated as a function of jet pT for two centrality intervals 0–10% and 30–40% is presented in the left panel of Fig. 2. The RAA value is obtained for jets with |y| < 2.1 and with pT between 80–1000 GeV. A clear suppression of jet production in central Pb+Pb collisions relative to pp collisions is observed. In the 0-10% centrality interval, RAA is approximately 0.45 at pT = 100 GeV, and is observed to grow slowly (quenching decreases) with increasing jet pT, reaching a value of 0.6 for jets with pT around√ 800 GeV. In the same figure, the RAA values at 5.02 TeV are compared with the previous measurements at sNN = 2.76 TeV. The two measurements agree within their uncertainties in the overlapping pT region. The apparent reduction of the size of systematic uncertainties in the new measurement is possible thans to large samples of pp and Pb+Pb data collected during the same LHC running period. Further insight in jet quenching can be obtained by studying jet fragmentation functions. The right panel of Fig. 2 presents a ratio of transverse jet fragmentation functions D(pT)inPb+Pb to those extracted in pp collisions as a function of jet fragment pT for three jet pT intervals. The RD(pT) is above unity (enhancement) for low pT jet fragments, drops below unity for intermediate jet pT fragments (suppression) and becomes ff larger than unity again for jet fragment pT around 50 GeV. There is no significant di erence between RD(pT) for three jet pT intervals. A comparison between the data and the hybrid model calculation is also shown. 10 I. Grabowska-Bold / Nuclear Physics A 982 (2019) 8–14 2.5 jet AA ATLAS anti-k R = 0.4 jets |y| < 2.1 ATLAS | | < 2.1 anti- =0.4 jets t y kt R R ) 126 < p jet < 158 GeV 0 - 10%, s = 2.76 TeV [PRL 114 (2015) 072302] T NN T p 200 < p jet < 251 GeV 0 - 10%, s = 5.02 TeV ( NN 2 T 1 jet 30 - 40%, s = 2.76 TeV [PRL 114 (2015) 072302] D 316 < p < 398 GeV NN T Hybrid Model, R = 3 30 - 40%, sNN = 5.02 TeV R res T and luminosity uncer. 126 < p jet < 158 GeV 〈 AA〉 T 200 < p jet < 251 GeV 1.5 T 316 < p jet < 398 GeV T 1 0.5 Pb+Pb, = 5.02 TeV, 0.49 nb-1, 0-10% sNN 0.5 pp , s = 5.02 TeV, 25 pb-1 2 40 60 100 200 300 500 900 1 10 10 p [GeV] p [GeV] T T Fig. 2. (Left) Inclusive jet RAA as a function of jet pT for jets with |y| < 2.1 in 0–10% and 30–40% centrality intervals compared to + the same quantity measured in 2.76 TeV Pb Pb collisions [5]. (Right) RD(pT) ratios for three jet pT ranges: 126–158 GeV (circles), 200–251 GeV (diamonds) and 316–398 GeV (crosses) compared with calculations from the hybrid model with Rres = 3 [6]. The model is able to describe the intermediate- and high-pT regions for jet fragments, while it fails in the low-pT region. The ATLAS Collaboration performed a preliminary measurement of the angular distribution of charged particles around the jet axis in 5.02 TeV Pb+Pb and pp data [7]. The measured yields are defined as: 2 = 1 1 d nch(r), RD(pT) (1) Njet 2πr drdpT π where Njetis the total number of jets, 2 rdr is the area of the annulus at a given distance r from the jet axis (r = Δη2 +Δφ2 with Δη and Δφ being the relative differences between the charged particle and the jet axis, in pseudorapidity and azimuth respectively), dr is the width of the annulus and nch(r) is the number of charged particles within a given annulus. Results are presented as a function of Pb+Pb collision centrality, and both jet and charged-particle pT in the left panel of Fig. 3. Ratios of D(pT, r) distributions in Pb+Pb to those measured in pp collisions as a function of r for six charged-particle pT intervals spanning values between 1.6–63.1 GeV in 0–10% centrality, and for jet pT between 200–251 GeV are shown. The RD(pT,r) is above unity for all r values for charged particles with pT less than 4 GeV. For these particles, RD(pT,r) grows < . < < . > . with increasing r for r 0 3 and is approximately constant for 0 3 r 0 6. For pT 4 0 GeV, RD(pT,r) is below unity and decreases with increasing r for r < 0.3 and is approximately constant for 0.3 < r < 0.6. The observed behaviour inside the jet (r < 0.4) agrees with the measurement of the inclusive jet fragmentation functions [6], where yields of the low-pT fragments are observed to be enhanced and yields of charged particles with intermediate pT are suppressed.
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